The present invention relates generally to methods and arrangements for controlling the operation of pixels in a display system having a certain frame rate. More specifically, the invention relates to using an analog signal to control the switching of binary pixels of a spatial light modulator between their two operating states such that the pixels produce modulated light having gray scale during each frame of the display system.
In the field of display systems and especially those using spatial light modulators having binary pixels that may only be switched between two states (i.e. an ON and an OFF state), it is known that stationary and moving images, either monochrome or color, may be sampled and both color-separated and gray-scale separated pixel by pixel. These pixelated separations may be digitized forming digitized images which correspond to the given images. These digitized images are used by spatial light modulators having binary pixels to create visual images that can be used for a direct visual display, a projected display, a printer device, or for driving other devices that use visual images as their input. One such novel image generator is disclosed in U.S. patent application Ser. No. 08/362,665, now U.S. Pat. No. 5,748,164 entitled ACTIVE MATRIX LIQUID CRYSTAL IMAGE GENERATOR, which application is incorporated herein by reference.
At present, when such binary-pixel spatial light modulators are used in gray-scale display systems, they are controlled by externally provided digital signals. These digital driving methods suffer from several shortcomings. First, in many cases the display input signal is an analog signal. This analog signal must be digitized in order to provide the drive signal needed by the individual pixels. This digitization step may introduce unwanted display system complexity in the form of analog-to-digital converters, frame buffer memories, etc. Further, the transmission of digital video signals requires a high bandwidth communication link to the display. This high bandwidth link may be expensive and may consume excessive electrical power. Second, the techniques used to address binary pixels with externally generated digital control signals (for example, as disclosed in the above-referenced U.S. patent application, Ser. No. 08/362,665) may require pixel switching times that are impractically fast to achieve a finely-gradated gray scale with digital drive of binary pixels. Both of these shortcomings may be overcome by providing methods and arrangements for controlling the switching of binary pixels using an analog signal.
The present invention discloses arrangements and methods for controlling the operation of binary pixels using an analog signal to control the gray scale level of each pixel rather than a sequence of digitized signals.
As will be described in more detail hereinafter, a method for operating a pixel and a pixel configuration for use in a display system is herein disclosed. A display system including pixels designed in accordance with the invention is also disclosed. The display system includes a spatial light modulator having an array of individually controlled pixels, such as binary pixels, switchable between a first and a second state. The spatial light modulator produces modulated light having gray scale during a given period of time. The pixel includes an arrangement for receiving a reference signal and an arrangement for receiving an analog pixel image signal. The reference signal is a signal that varies in a predetermined way during the given period of time. The analog pixel image signal is a signal representing a desired gray scale level for the pixel during the given period of time. The pixel also includes a comparator for comparing the reference signal and the analog pixel image signal and outputting a signal for switching the pixel between the first and the second state when the reference signal reaches a predetermined level relative to the analog pixel image signal.
In one embodiment, the reference signal is a signal having a voltage that varies in a predetermined way during the given period of time and the analog pixel image signal is a voltage representing the desired gray scale level for the pixel during the given period of time. For example, the voltage of the reference signal may vary linearly throughout the given period of time. In this embodiment, the pixel further includes a storing arrangement, such as a capacitor, for storing the analog pixel image signal voltage.
In another embodiment, the comparing arrangement includes a comparator circuit for outputting a binary output signal. The pixel further includes an inverter arrangement for inverting the output of the comparing arrangement. In a specific version of this embodiment, the pixel includes a liquid crystal light modulating medium that requires DC-field balancing in order to prevent the degradation of the liquid crystal light modulating medium. Also, the reference signal is a signal that varies in a predetermined way and in the same manner during a first and a second equal portion of the given period of time. The pixel further includes an arrangement for activating the inverter arrangement during the second portion of the given period of time. This causes the inverter arrangement to invert the output of the comparing arrangement during the second portion of the given period of time and automatically DC-field balances the liquid crystal light modulating material during the given period of time without requiring the pixel to receive any additional pixel switching data during the given period of time.